The invention relates to a method for decontaminating a boiling water nuclear reactor employed to generate electric power and more particularly to a method for decontaminating one or more external recirculation loops hydraulically connected with a reactor pressure vessel without decontaminating the reactor pressure vessel.
During on-line power generating operations of commercial boiling water nuclear reactors, thin metal oxide layers or films tend to build up on the internal surfaces of the reactor pressure vessels and appurtenant subsystems in contact with recirculating coolant water. These reactors commonly have at least one and generally two special piping subsystems, commonly referred to collectively as a xe2x80x9creactor recirculation systemxe2x80x9d (or xe2x80x9cRRSxe2x80x9d) and individually as xe2x80x9crecirculation loopsxe2x80x9d (or xe2x80x9cloopsxe2x80x9d), piped in parallel with each other and hydraulically connected to a reactor pressure vessel to assist the recirculation of the coolant water inside the reactor pressure vessel during power generation. Activated metal ions from the central core regions within the reactor pressure vessels tend to dissolve in the high velocity recirculating coolant water and then to become absorbed in the metal oxide layers throughout the balance of the reactor system, which tends to raise the overall radiation levels throughout the entire reactor system.
Later, during periodic scheduled outages, it is desirable to reduce the general radiation levels within containment buildings in which the nuclear reactors are contained to reasonably achievable levels in order to reduce the potential exposure of personnel working within the buildings during the periodic outages. The nuclear industry has developed various dilute chemical decontamination treatments, e.g., commercial treatments such as AP-Citrox, AP-AC, CAN-DEREM, LOMI and the like, for dissolving the metal oxide layers and thereby reducing radiation levels of boiling water reactors and pressurized water reactors. Conventionally, these decontamination processes involve the addition of an oxalate, a citrate, EDTA, low oxidation state metal ion and the like to the coolant water to form decontamination solutions, which are then pumped throughout the portions of the reactor system to be decontaminated to dissolve and solubilize the activated ions absorbed in the oxide layers. The metal ions are then removed from the decontamination solutions on ion exchange resins. In addition, particulate oxides from the layers may be entrained by the decontamination solutions and then removed on filters. See, generally, M. E. Pick et al., xe2x80x9cChemical Decontamination Of Water Reactors. CEGB Developments And The International Scenexe2x80x9d, Nuclear Energy, Vol. 22, No. 6, pp. 433-444 (December 1983).
In full loop decontamination processes, only the external recirculation loops hydraulically connected with the reactor pressure vessels of boiling water reactors are normally decontaminated. Decontamination of the loops is desirable because the portions of the reactors located outside of the reactor pressure vessels tend to be major contributors to the radiation dose rates in the containment buildings. It is frequently not necessary to decontaminate the internal portions of the reactor pressure vessels because the radiation levels within the reactor pressure vessels, although very high, do not contribute to the dose rates in the general vicinity of the containment buildings where the maintenance personnel and their equipment will be located. Also, it is frequently not desirable to decontaminate the internal portions of the reactor pressure vessels in order to reduce the amount of radioactive material that is handled and eventually removed and stored.
Prior to performing a chemical decontamination of a particular portion of a system, the subsystem or component to be decontaminated is normally isolated from the balance of the system in order to confine the decontamination solution to the desired portion of the system. Thus, in accordance with known and proposed prior practices for performing full loop decontaminations without decontaminating the hydraulically connected nuclear pressure vessels, the loops may be isolated from the reactor pressure vessels by one of three preliminary methods: (1) removing all of the nuclear fuel from the reactor pressure vessels and then draining the reactor pressure vessels down to levels where the water levels are below the levels of the nozzles in the reactor pressure vessels connected with the external loop piping; (2) installing specially designed cross-connections (sometimes referred to as xe2x80x9cjumpersxe2x80x9d) between the internal jet pump risers within the reactor pressure vessels; or (3) installing plugs into the reactor pressure vessel nozzles connected with the external loop piping.
Of these three preliminary methods for isolating the loops from the reactor pressure vessels, the use of plugs in accordance with preliminary method (3) is often the method of choice because it has much less impact on reactor outage critical path time and results in the least personnel exposure of personnel to radiation. However, during the course of the subsequent loop decontamination steps when the decontamination solutions are pumped through the loops, the levels of the decontamination solutions should be closely monitored in order to determine whether the decontamination solutions have sufficiently washed the irradiated surfaces of, and thereby effectively decontaminated, the loop piping. A lack of effective methods for monitoring the decontamination solution levels inside the loop piping and the inability to establish reliable vent paths for the gases trapped within the loops adjacent the plugs has caused prior loop decontamination processes performed with plugged nozzles to be significantly less effective than decontamination processes performed with the other two preliminary methods.
It is an object of the present invention to provide a method for decontaminating the recirculation loops of boiling water reactors that will permit workers to monitor the levels of decontamination solutions in loops isolated by plugs from reactor pressure vessels with greater accuracy than was provided in prior loop decontamination practices. It is a further object to provide a decontamination method that will provide a reliable vent path for the gases trapped inside the plugged loop piping.
With these objects in view, the present invention resides in a method of decontaminating a boiling water reactor comprising at least one recirculation loop having a plurality of recirculation pipes piped in parallel and hydraulically connected with a reactor pressure vessel. In the general practice of the present invention, plugs are installed on the outlets of a jet pump ram""s head manifold connected with a riser pipe extending from a reactor pressure vessel nozzle connected with one of the parallel recirculation pipes in the recirculation loop. Preferably, the outlets of all of the manifolds in the recirculation loop are plugged.
A monitoring gas is introduced into the ram""s head manifold through one of the plugs installed therein for monitoring the pressure in the manifold connected with the riser pipe. The level of a decontamination solution in the recirculation loop is determined from the pressure of the monitoring gas in the manifold connected with the riser pipe. The monitoring gas need only be introduced into one of the loops through one of the outlets of one of the ram""s head manifolds in the loop. However, it is preferable to introduce monitoring gas through the outlets of at least two manifolds in each of the loops.
Advantageously, the monitoring gas may be introduced into the manifold by a suitable commercially available level/pressure monitoring system such a gas bubbler. This type of monitoring system can sense the pressure and change in pressure of the monitoring gas introduced into a space and provide a corresponding signal to a programmable logic controller or other calculating device. Based upon signals indicating the pressure of the monitoring gas in the manifold (and the pressure of the monitoring gas introduced into the loop at a lower location), the controller can calculate the level of the decontamination solution in the loop from the differential pressure due to the height (or head) of the decontamination solution in the loop.
In preferred practices where the monitoring gas is introduced into one or more manifolds in accordance with the present invention, the decontamination solution may be pumped in, into or out of the riser pipes while the monitoring gas is being introduced into the ram""s head manifolds. In one preferred practice, the decontamination solution may be sloshed in the risers while introducing the monitoring gas into the ram""s head manifolds. In preferred practices of the present invention in connection with boiling water reactors (such as commercial boiling water reactors for generating electric power) having at least two recirculation loops, the decontamination solution may be pumped from one of the two loops to the other of the two loops while introducing the monitoring gas into the ram""s head manifold(s) of one or both loops.
In a preferred practice of the present invention where it is desired to provide a reliable venting path from a ram""s head manifold, the monitoring gas is vented from the ram""s head manifold through one of the plugs installed therein. Preferably, the monitoring gas is introduced into the manifold through one plug and then vented through the other plug in the manifold. Advantageously, the monitoring gas and the gas trapped in the manifold when the decontamination solution is pumped or sloshed through the riser pipes and the balance of the loop may vent through the plug. Most preferably, each of the manifolds in a loop will have a vent for venting the monitoring gases. Also, the gases may be vented while the decontamination solution is being pumped or sloshed in a loop or between loops.
In a preferred practice of the present invention where one or more of the manifolds are vented, a vacuum is pulled on the manifolds. Advantageously, a vacuum will suck any water trapped in the plugged outlets of the manifolds that would prevent the flow of gas from the manifolds.
In a preferred practice of the present invention where it is desired to determine the level of the decontamination solution in the suction piping of a recirculation pump in the loop, a plug is installed in a reactor vessel nozzle connected with a suction pipe extending to the suction connection of the recirculation pump. A monitoring gas is introduced through the plug in the suction pipe extending to the suction connection of the recirculation pump for monitoring the pressure in the suction pipe connected with the suction connection of the recirculation pump. The level of the decontamination solution in the suction pipe extending to the suction connection of the recirculation pump is determined from the pressure of the monitoring gas introduced through the plug in the reactor vessel nozzle into the suction pipe extending to the suction connection of the recirculation pump.